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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{derived category} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{homological_algebra}{}\paragraph*{{Homological algebra}}\label{homological_algebra} [[!include homological algebra - contents]] \hypertarget{homotopy_theory}{}\paragraph*{{Homotopy theory}}\label{homotopy_theory} [[!include homotopy - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{history}{History}\dotfill \pageref*{history} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{Properties}{Equivalent characterizations and constructions}\dotfill \pageref*{Properties} \linebreak \noindent\hyperlink{AsLOcalizationAtNullSystem}{In terms of localization at a null system}\dotfill \pageref*{AsLOcalizationAtNullSystem} \linebreak \noindent\hyperlink{InTermsOfResolutions}{In terms of injective and projective resolutions}\dotfill \pageref*{InTermsOfResolutions} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The \emph{derived (infinity,1)-category} or \emph{derived category} of an [[abelian category]] $\mathcal{A}$ is the setting for [[homological algebra]] in $\mathcal{A}$: the [[(infinity,1)-categorical localization]] of the [[category of chain complexes]] in $\mathcal{A}$ at the class of [[quasi-isomorphisms]]. The derived category is a fundamental example of a [[stable (infinity,1)-category]]. By the [[stable Dold-Kan correspondence]], it may be viewed as a linearization of the [[stable (infinity,1)-category of spectra]]. The derived (infinity,1)-category is presented by various [[dg-model structures]] on the [[category of chain complexes]], as described at [[model structures on chain complexes]]. As such it has also a natural incarnation as a [[pretriangulated dg-category]], which might be called the \emph{derived dg-category}. Like any [[stable (infinity,1)-category]], the [[homotopy category of an (infinity,1)-category|homotopy category]] of the derived (infinity,1)-category admits a canonical [[triangulated category]] structure. Often in the literature, the term \emph{derived category} refers to the [[homotopy category of an (infinity,1)-category|homotopy category]], viewed only as a [[triangulated category]]. The loss of information can often be problematic, but for many purposes is not important. In what follows, we will describe only the homotopy category. See [[(infinity,1)-category of chain complexes]] for the full [[(infinity,1)-category]]. Associated to $\mathcal{A}$ is \begin{itemize}% \item the [[category of chain complexes]] $Ch_\bullet(\mathcal{A})$ in $\mathcal{A}$ which is naturally a [[homotopical category]]; \item the ``[[homotopy category of chain complexes]]'' $K(\mathcal{A})$; \item the [[stable ∞-category]] $K_\infty(\mathcal{A})$ of [[chain complexes]] in $C$. \end{itemize} The \emph{derived category} $D(C)$ of $C$ is equivalently \begin{itemize}% \item the [[homotopy category|1-categorical homotopy category]] of $Ch_\bullet(\mathcal{A})$ with respect to the [[quasi-isomorphisms]]; \item the [[homotopy category of an (infinity,1)-category|(∞,1)-categorical homotopy category]] of $K_\infty(\mathcal{A})$. \end{itemize} In either case, this means that under the canonical [[localization]] functor \begin{displaymath} Q : Ch_\bullet(\mathcal{A}) \to D(\mathcal{A}) \end{displaymath} the [[quasi-isomorphisms]] of [[chain complexes]] become true [[isomorphisms]] and that $D(\mathcal{A})$ is [[universal property|universal]] with respect to this property. Hence the derived category is an approximation to the full [[simplicial localization]] of $K(\mathcal{A})$. It is or can be equipped with several further [[properties]] and [[structure]] that give a more accurate approximation. Notably every derived category is a \emph{[[triangulated category]]}, which is a way of remembering the [[suspension]] and de-suspension operations on its objects -- the [[suspension of chain complexes]] -- hence its ``[[stable (infinity,1)-category|stability]]''. \hypertarget{history}{}\subsection*{{History}}\label{history} Derived categories were introduced by [[Jean-Louis Verdier]] in his thesis under the supervision of [[Alexandre Grothendieck]]. It was originally used to extend [[Serre duality]] to a relative context. See [[Robin Hartshorne|Hartshorne]]`s lecture notes ``Residues and duality''. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} Let $\mathcal{A}$ be an [[abelian category]] and $K(\mathcal{A})$ its [[category of chain complexes]] modulo [[chain homotopy]] (the ``[[homotopy category of chain complexes]]''). Equip $K(\mathcal{A})$ with the structure of a [[homotopical category]] by declaring the [[weak equivalences]] to be the \textbf{quasi-isomorphisms}: those morphisms $f : V \to W$ which induce [[isomorphisms]] in [[homology]], $H(f) : H(V) \stackrel{\simeq}{\to} H(W)$. \begin{defn} \label{}\hypertarget{}{} The \textbf{derived category} $D(\mathcal{A})$ is the [[homotopy category]] of $K(\mathcal{A})$ with respect to these weak equivalences. \end{defn} Analogously, for $K^{+,-,b}(\mathcal{A})$ denoting the [[full subcategory]] on the chain complexes bounded above, bounded below, or bounded, respectively (see at \emph{[[category of chain complexes]]}), one writes \begin{displaymath} D^{+,-,b}(\mathcal{A}) \hookrightarrow D(\mathcal{A}) \end{displaymath} for the correspponding full subcategory of the derived category. \hypertarget{Properties}{}\subsection*{{Equivalent characterizations and constructions}}\label{Properties} There are various ways to construct or express the derived category more explicitly in terms of various special objects or morphisms in the [[category of chain complexes]]. \begin{itemize}% \item \emph{\hyperlink{AsLOcalizationAtNullSystem}{In terms of localization at a null system}} \item \emph{\hyperlink{InTermsOfResolutions}{In terms of resolutions}}. \end{itemize} \hypertarget{AsLOcalizationAtNullSystem}{}\subsubsection*{{In terms of localization at a null system}}\label{AsLOcalizationAtNullSystem} The ``[[homotopy category of chain complexes]]'' $K(\mathcal{A})$ is already a [[triangulated category]]. The derived category can be obtained as the construction of a [[homotopy category]] of a [[triangulated category]] with respect to a [[null system]]. \begin{defn} \label{}\hypertarget{}{} Let \begin{displaymath} N(\mathcal{A}) \subset K(\mathcal{A}) \end{displaymath} and analogously \begin{displaymath} N^{+,-n,b}(\mathcal{A}) \subset K^{+,-,b}(\mathcal{A}) \end{displaymath} be the [[full subcategory]] of $K(C)$ or on $K^{+,-,b}$, respectively, on those [[chain complexes]] $V$ whose [[chain homology]] vanishes in every degree, $H_\bullet(V) = 0$. \end{defn} \begin{prop} \label{}\hypertarget{}{} A [[chain map]] $f_\bullet : V_\bullet \to W_\bullet$ is a [[quasi-isomorphism]] precisely there exists a [[distinguished triangle]] in $K(\mathcal{A})$ of the form \begin{displaymath} V \stackrel{f}{\to} W \to cone(f) \end{displaymath} with the [[mapping cone]] $cone(f) \in N(\mathcal{C})$. \end{prop} \begin{prop} \label{}\hypertarget{}{} The derived category is equivalently the localization of $K(\mathcal{A})$ at the [[null system]] $N(\mathcal{A})$. \begin{displaymath} D(\mathcal{A}) \simeq K(\mathcal{A})/N(\mathcal{A}) \,. \end{displaymath} \end{prop} This perspective is discussed in (\hyperlink{KashiwaraSchapira}{Kashiwara-Schapira, section 13}) and (\hyperlink{Schapira}{Schapira, section 6.2, 72}). \hypertarget{InTermsOfResolutions}{}\subsubsection*{{In terms of injective and projective resolutions}}\label{InTermsOfResolutions} In the case that the underlying [[abelian category]] $\mathcal{A}$ has \href{injective%20object#EnoughInjectives}{enough injectives} or \href{projective%20object#EnoughInjectives}{enough projectives}, the [[hom sets]] in the derived category may equivalently be obtained as [[homotopy]]-classes of [[chain maps]] from [[projective resolutions]] to [[injective resolutions]] of chain complexes. In view of the existence of the injective and projective [[model structure on chain complexes]] this is a special case of the general fact that [[homotopy categories]] of [[model categories]] may be obtained by forming homotopy classes of maps in the model category from [[cofibrant resolutions]] to [[fibrant resolutions]]. But here we spell out an direct discussion of this fact for chain complexes. \begin{defn} \label{}\hypertarget{}{} Write $K^+(\mathcal{I}_{\mathcal{A}}) \hookrightarrow K^+(\mathcal{A})$ for the [[full subcategory]] of the [[homotopy category of chain complexes]] bounded above on those that are degreewise [[injective objects]]. Dually, let $K^-(\mathcal{P}_{\mathcal{A}}) \hookrightarrow K^-(\mathcal{A})$ for the [[full subcategory]] of the [[homotopy category of chain complexes]] bounded below on those that are degreewise [[projective objects]]. \end{defn} \begin{theorem} \label{}\hypertarget{}{} If $\mathcal{A}$ has \href{http://ncatlab.org/nlab/show/injective%20object#EnoughInjectives}{enough injectives} then the canonical functor \begin{displaymath} K^+(\mathcal{I}_{\mathcal{A}}) \to D^+(\mathcal{A}) \end{displaymath} is an [[equivalence of categories]]. Dually, if $\mathcal{A}$ has \href{http://ncatlab.org/nlab/show/projective%20object#EnoughProjectives}{enough projectives} then the canonical functor \begin{displaymath} K^-(\mathcal{P}_{\mathcal{A}}) \to D^-(\mathcal{A}) \end{displaymath} is an [[equivalence of categories]]. \end{theorem} For instance (\hyperlink{Schapira}{Schapira, cor. 7.3.2}). \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[derived functor]] \item [[triangulated categories]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} The original reference is the thesis of [[Verdier]]: \begin{itemize}% \item [[Verdier, Jean-Louis]], \emph{Des Cat\'e{}gories D\'e{}riv\'e{}es des Cat\'e{}gories Ab\'e{}liennes}, Ast\'e{}risque (Paris: Soci\'e{}t\'e{} Math\'e{}matique de France) 239. Available in \href{http://www.math.jussieu.fr/~maltsin/jlv.html}{electronic format} courtesy of [[Georges Maltsiniotis]]. \end{itemize} A systematic discussion from the point of view of [[localization]] and [[homotopy theory]] is in section 13 of \begin{itemize}% \item [[Masaki Kashiwara]], [[Pierre Schapira]], \emph{[[Categories and Sheaves]]} \end{itemize} and, similarly, in section 7 of \begin{itemize}% \item [[Pierre Schapira]], \emph{Categories and homological algebra} (2011) (\href{http://people.math.jussieu.fr/~schapira/lectnotes/HomAl.pdf}{pdf}) \end{itemize} A pedagogical introduction is \begin{itemize}% \item R. P. Thomas, \emph{Derived categories for the working mathematician} (\href{http://arxiv.org/abs/math.AG/0001045}{arXiv:math.AG/0001045}) \end{itemize} A good survey of the more general topic of derived categories is \begin{itemize}% \item [[Bernhard Keller]], \emph{Derived categories and their uses} (\href{http://www.google.de/url?sa=t&source=web&ct=res&cd=6&ved=0CC8QFjAF&url=http%3A%2F%2Fwww.math.jussieu.fr%2F~keller%2Fpubl%2Fdcu.ps&rct=j&q=derived+category&ei=Ib76SsSdAsjb-QaAw7moDw&usg=AFQjCNGIgXLHlprAoR70bGLWQmyKGHDjTQ}{ps}) \end{itemize} See in particular also the list of references given there. Other lecture notes include \begin{itemize}% \item Theo B\"u{}hler, \emph{An introduction to the derived category} (\href{http://www.uni-math.gwdg.de/theo/intro-derived.pdf}{pdf}) \end{itemize} For a discussion in the context of [[(∞,1)-category|(∞,1)-categories]] and in particular [[stable (∞,1)-category|stable (∞,1)-categories]] see \href{http://www.math.harvard.edu/~lurie/papers/DAG-I.pdf#page=53}{section 13, p. 53} of \begin{itemize}% \item [[Jacob Lurie]], \emph{[[Stable ∞-Categories]]} \end{itemize} For the applications of derived categories in [[algebraic geometry]], see \begin{itemize}% \item [[Dmitri Orlov]], \emph{Derived categories of coherent sheaves and equivalences between them}, Uspekhi Mat. Nauk, 2003, Vol. 58, issue 3(351), pp. 89--172, \href{http://www.mi.ras.ru/~orlov/papers/Uspekhi2003.pdf}{English translation (PDF)} \item [[Aleksei Bondal]], [[Dmitri Orlov]], \emph{\href{http://www.mi.ras.ru/~orlov/papers/Compositio2001.pdf}{Reconstruction of a variety from the derived category and groups of autoequivalences}}, Compositio Mathematica 125 (03), 327-344. See also [[Bondal-Orlov reconstruction theorem]]. \item [[Daniel Huybrechts]], \emph{Fourier-Mukai Transforms in Algebraic Geometry}, Oxford University Press, USA, 2006. \item [[Igor Dolgachev]], \href{http://www.math.lsa.umich.edu/~idolga/derived9.pdf}{\emph{Derived categories}}. \item [[Andrei Caldararu]], \emph{Derived categories of coherent sheaves: a skimming}. Lecture notes from \emph{Algebraic Geometry: Presentations by Young Researchers} in Snowbird, Utah, July 2004. Available on \href{http://arxiv.org/abs/math/0501094}{arXiv}. \end{itemize} [[!redirects derived categories]] [[!redirects bounded derived category]] [[!redirects bounded derived categories]] [[!redirects derived (infinity,1)-category]] [[!redirects derived (infinity,1)-categories]] [[!redirects derived dg-category]] [[!redirects derived dg-categories]] \end{document}